The present invention generally relates to an error signal generator for use in an optical disk drive. Specifically, it relates to an error signal generator of an optical disk, which generator generates error signals by performing arithmetic operations on output signals from a light detecting means consisting of a plurality of photodiodes (light-sensitive elements).
An error signal generator for use in an optical disk drive, which generator uses a micro-prism detector shown in FIG. 1A, is known in prior art.
Referring to FIG. 1A, a laser beam 4 radiated from a laser diode 2 is turned into parallel rays by a collimator lens 5 before it is admitted into a reflecting prism 7 via a beam splitter 6. After being reflected by the prism 7, the laser beam 4 is made to converge by an object lens 8 and is then incident on a recordable side of an optical disk 1.
The laser beam 4 which is incident on the optical disk 1 is reflected by a reflecting film formed on the optical disk 1. The laser beam 4 reflected thereon is turned into parallel rays by the object lens 8 before it is reflected by the reflecting prism 7 and is incident on the beam splitter 6 to be reflected in the downward direction in the figure. After being passed through a half-wave plate 10, this first reflected light 9 is made to converge by a plano-convex lens 11 (converging means) on a first focal point 12.
Along the optical path of the first reflected light 9 is disposed a micro-prism detector 3 (hereinafter called an MPD) consisting of a polarizing beam splitter 13a (light splitting means) provided by forming a specified polarizing film on the overlapping portion of two prisms 13 and 14 mated with each other, the prisms 13 and 14, and light detectors 15 and 16. As the first reflected light 9 travels to the first focal point 12, it is also split by the polarizing beam splitter 13a and reflected by surface 14a of the prism 14. This second reflected light 17 is convergent at on a second focal point 18.
Referring to FIG. 1A, the light detector 15 (first light detecting means), disposed between the first focal point 12 and the polarizing beam splitter 13a, detects the first reflected light 9. In the figure, the light detector 16 (second light detecting means), disposed below the second focal point 18, detects the second reflected light 17.
The light detector 15 consists of a plurality of light-sensitive elements in the form of photodiodes 15a, 15b, and 15c, as shown in FIG. 1B. The photodiodes 15a and 15c are manufactured such that they are roughly of a semicircular shape. These photodiodes 15a and 15c constitute a circular light detector 15 by sandwiching a photodiode 15b of a roughly rectangular shape across respective boundaries 19a and 19b.
The light detector 15 is formed such that it has a symmetry with respect to a central line 19c. The light detector 15 is disposed such that the center of the first reflected light 9 is incident on the central line 19c.
The light detector 16 also consists of a plurality of light-sensitive elements in the form of photodiodes 16a, 16b, and 16c, and is configured in the same way as the light detector 15. The light detector 16 is disposed such that the center of the second reflected light 17 is incident on a central line 19d.
Outputs of the photodiodes included in the light detectors 15 and 16 are supplied to an operating means 20.
In an error signal generator for use in a conventional optical disk drive of the above-mentioned configuration, the operating means 20 performs specified arithmetic operations on the outputs of the photodiodes of the light detectors 15 and 16. An RF signal is obtained by a differential detection method, a focus error signal is obtained by a beam size detection method, and a tracking error signal is obtained by a sampling servo method.
A description of the principles of a beam size detection method will be given in accordance with FIGS. 2A through 2C, and 3A through 3C. In the figures, optical systems are drawn in a simplified manner. In FIGS. 2A through 2C, and 3A through 3C, components that are the same as the components of the light detectors shown in FIGS. IA and 1B, are given the same reference notation, and the descriptions thereof are omitted.
FIGS. 2A and 3A show a condition in which the object lens 8 and the optical disk 1 are too close for the laser beam to be focused on the recordable side of the optical disk 1. The second reflected light 17 detected by the light detector 16 is smaller in area than the first reflected light 9 detected by the light detector 15. The regions on the photodiodes 15 and 16 illuminated by the first and second reflected lights 9 and 17, respectively, are shown as circle areas and hatched areas in the figures, the former representing areas illuminated by the 0 order diffracted light, and the latter representing areas illuminated by the .+-.1 order diffracted light.
FIGS. 2B and 3B show a condition in which the distance between the object lens 8 and the optical disk 1 is the specified distance, and thus the light is focused. In this case, the area illuminated by the first reflected light 9 as it is incident on the light detector 15, and the area illuminated by the second reflected light 17 as it is incident on the light detector 16 are the same.
FIGS. 2C and 3C show a condition in which the distance between the object lens 8 and the optical disk 1 is longer than the specified distance, and thus the light is not focused. In this case, the area illuminated by the second reflected light 17 as it is incident on the light detector 16 is larger than the area illuminated by the first reflected light 9 as it is incident on the light detector 15.
Specified arithmetic operations are then performed on outputs from a plurality of photodiodes constituting each of the light detectors 15 and 16. These operations allow detection of the difference between the dimensions of the area illuminated by the first reflected light 9 and that illuminated by the second reflected light 17 as the lights are incident on the light detectors 15 and 16. These operations enable the obtaining of an error signal that indicates an error of the laser beam focus with respect to the specified illumination condition on the optical disk 1. This method is called beam size detection method because a focus error signal is obtained from the difference in the dimensions of illumination areas of each of the reflected lights 9 and 17 on each of the light detectors 15 and 16.
Given that output currents of each of the photodiodes 15a, 15b, 15c, 16a, 16b and 16c are I.sub.15a, I.sub.15b, I.sub.15c, I.sub.16a, I.sub.16b, and I.sub.16c respectively, a focus error signal FE that indicates the error of focal position of the laser beam along the direction of the illumination can be obtained as follows; EQU FE=I.sub.15a +I.sub.15c +I.sub.16b -(I.sub.15b +I.sub.16a +I.sub.16c) (1)
While the dimensions of the areas illuminated by the reflected light 9 and 17 are different between FIGS. 3A and 3C, the total luminous energy received by each photodiode is approximately the same in the two conditions. Thus, the total luminous energy received by each of the photodiodes 15a-15c constituting the light detector 15, and the total luminous energy received by the photodiodes 16a-16c constituting the light detector 16 are approximately the same. On the other hand, as the illumination area on each light detector varies, the ratio of luminous energy received by each of three photodiodes 15a-15c and each of three photodiodes 16a-16c, constituting the light detectors respectively, varies. The formula (1) above is used in obtaining the ratio of luminous energy received by each of the photodiodes within the same light detector.
As is generally known, a tracking error signal TE, obtained by a sampling servo method to be described later, is given by EQU TE=I.sub.15a +I.sub.15b +I.sub.15c ( 2)
or EQU TE=I.sub.16a +I.sub.16b +I.sub.16c ( 2)'
or EQU TE=I.sub.15a +I.sub.15b +I.sub.15c +I.sub.16b +I.sub.16c ( 2)"
A tracking error signal is defined as a signal that indicates an error, with respect to the center, in the direction of the tracking by a laser beam. A sampling servo method can be performed on an optical disk (having a sampling format) manufactured so that a pair of pits are provided separately on both sides of each information recording track. By comparing the intensity of light reflected from these pits, a tracking error signal is obtained.
An RF (Radio Frequency) signal corresponding to the information recorded on the optical disk 1 can be obtained by determining the difference between the outputs of each of the light detectors 15 and 16. That is, it is calculated as per the following equation. EQU RF=I.sub.15a +I.sub.15b +I.sub.15c -(I.sub.16a +I.sub.16b +I.sub.16c) (3)
As is generally known, this calculation is arrived at because in reproducing RF signals of a magneto-optical disk (MO disk), two prisms 13 and 14, which constitute the MPD 3, are configured such that the two kinds of reflected light 9 and 17, resulting from being passed through these prisms 13 and 14, have different polarization components and contain RF signal components that are 180 degrees out of phase.
The above-mentioned focus error signal, tracking error signal, and RF signal are obtained by means of an arithmetic circuit (operating means) comprising, for example, OP amplifiers (Operational Amplifiers).
A description will be given below of an arithmetic circuit in accordance with FIG. 4. In FIG. 4, components that are the same as the components of the light detectors 15 and 16 shown in FIGS. 3A through 3C are given the same reference notation and the descriptions thereof are omitted.
As can be seen in the figure, each of the output currents I.sub.15a, I.sub.15b, I.sub.15c, I.sub.16a, I.sub.16b, and I.sub.16c from each of the light detectors 15 and 16, are supplied to an arithmetic circuit 23 (operating means) comprising a plurality of OP amplifiers. The specified operations shown in the formulas (1), (2), and (3) are performed, a focus error signal is output to a terminal 24, a tracking error signal obtained by sampling servo method is output to a terminal 25, and an RF signal is output to a terminal 26.
In accordance with the above-mentioned error signal generator, of an optical disk drive equipped with an MPD, for generating a tracking error signal by means of a sampling servo method, and for generating a focus error signal and an RF signal, no special optical parts are necessary. Special optical parts in this case include a cylindrical lens needed in an astigmatic focus error signal generating method and an edge prism needed in Foucault's method of focus error signal generating.
The prisms 13 and 14 constituting an MPD 3 are, like the other prisms used in an optical disk drive, easily obtainable and inexpensive compared with such special optical parts as a cylindrical lens or an edge prism. Therefore, using an MPD provides an advantage that an optical system can be configured from inexpensive optical parts.
The conventional error signal generating method using an MPD is, as described above, an effective technology when a sampling servo method, which is applied to an optical disk having a sampling format, is employed in the generation of a tracking error signal. However, when a push-pull method (described later), which is applied to an optical disk having a pregroove format, is employed in generating a tracking error signal, obtaining a tracking error signal with a good linearity is difficult. Pregroove formatting of an MO disk means forming, at the time of manufacturing, a spiral-shaped continuous pregroove so that it accommodates, at a specified interval, address information that indicates position (address) on a disk.
It is provided by standards that a general-purpose 3.5 inch MO (Magneto Optics) disk should be pregroove formatted. For a 3.5 inch pregroove formatted MO disk, a tracking error signal needs to be generated by means of a push-pull method.
A description will be given below, with reference to FIG. 5 and FIGS. 6A through 6C, of a tracking error signal generating method using a push-pull method. Referring to FIG. 5, an optical disk 1 is pregroove formatted. That is, grooves 1a, 1b, 1c, 1d, and le are formed on the optical disk 1. Reflected light 9 of a laser beam reflected from the groove 1a, after being turned into parallel rays by an object lens not shown in the figure, is made to converge by a plano-convex lens 11 and is incident on a light detector 27.
The light detector 27 consists of two symmetrical photodiodes 27a and 27b, positioned opposite to each other. Outputs of the photodiodes 27a and 27b are input into an OP amplifier 28. This OP amplifier 28 calculates the difference between the outputs of each of the photodiodes 27a and 27b, thus allowing a generation of a tracking error signal.
FIG. 6A shows a condition in which a laser beam 9 illumination area on the optical disk 1 is displaced toward the left of the groove 1a in the figure (such a displacement of the laser beam 9 with respect to the direction of tracking shall be called a tracking displacement hereafter). FIG. 6C shows a condition in which the laser beam 9 illumination area is displaced toward the right of the groove 1a in the figure. Broken lines in FIGS. 6A through 6C represent optical paths and illumination areas, on the light detector 27, of the .+-.1 order diffracted light of the laser beam 9. The hatched areas represent illumination areas of .+-.1 order diffracted light within illumination areas of 0 order diffracted light resulting from the laser beam 9 as it is incident on the light detector 27.
In the conditions shown in FIGS. 6A and 6C, the illumination areas, on the light detector 27, of these .+-.1 order diffracted lights 9a and 9b, are asymmetric with respect to a boundary 27c on the photodiodes 27a and 27b. Therefore, a tracking error signal obtained through a subtractive operation by the OP amplifier 28, as described above, on the outputs of the photodiodes 27a and 27b shows either a positive or a negative value.
FIG. 6B shows a condition in which the laser beam 9 illumination area on the optical disk 1 is aligned with the center of the tracking direction, that is, the laser beam 9 center is incident on the center of the groove 1a. The light detector 27 is disposed so that this illumination area, on the light detector 27, illuminated by the .+-.1 order diffracted lights 9a and 9b resulting from the laser beam 9, is symmetrical with respect to the boundary 27c. Under this condition, the tracking error signal output from the OP amplifier operation is nil.
In accordance with the above-mentioned push-pull method, the light detector is disposed so that the center of the light reflected from the optical disk having a pregroove format hits the boundary of the photodiodes on the light detector split symmetrically into two photodiodes. An output signal in proportion to the dimensions of the illumination area on the light detector, is output from each of the above-mentioned photodiodes to the OP amplifier. The OP amplifier calculates the difference between the output signals so that a tracking error signal is obtained. This push-pull method is applied to a 3.5 inch MO disk, which is required by standards to have a pregroove format.
However, in an MPD of a conventional error signal generator of an optical disk drive, the photodiodes 15b and 16b roughly of rectangular shape are located in the center of each of the light detectors. Further, the first and second reflected lights are incident so that the center of the first and second reflected light beams hit the central line of each of the photodiodes 15b and 16b respectively. Thus, the generation of a tracking error signal in accordance with the push-pull method requires that the photodiodes 15a, 15c, 16a, and 16c, in exclusion of 15b and 16b, be used to obtain the difference between the two symmetrical regions. This has an adverse effect in that linearity of a tracking error signal thus obtained is degraded due to the fact that luminous energy of the illumination on the above-mentioned roughly rectangular photodiodes 15b and 16b are excluded from the calculation.